As practiced commercially, fused silica optical members such as lenses, prisms, filters, photomasks, reflectors, etalon plates and windows, are typically manufactured from bulk pieces of fused silica made in large production furnaces. Bulk pieces of fused silica manufactured in large production furnaces are known in the art as boules or ingots. Blanks are cut from boules or ingots, and finished optical members are manufactured from glass blanks, utilizing manufacturing steps that may include, but are not limited to, cutting, polishing, and/or coating pieces of glass from a blank. These optical members are used in various apparatus employed in environments where they are exposed to high-power ultraviolet light having a wavelength of about 360 nm or less, for example, an excimer laser beam or some other high-power ultraviolet laser beam. The optical members are incorporated into a variety of instruments, including lithographic laser exposure equipment for producing highly integrated circuits, laser fabrication equipment, medical equipment, nuclear fusion equipment, or some other apparatus which uses a high-power ultraviolet laser beam.
In overview, boules are manufactured by reacting silicon-containing gas molecules in a flame to form silica soot particles. The soot particles are deposited on the hot surface of a rotating or oscillating body where they consolidate to the glassy solid state. In the art, glass making procedures of this type are known as vapor phase hydrolysis/oxidation processes, or simply as flame deposition processes. The term “boule” is used herein with the understanding that the term “boule” includes any silica-containing body formed by a flame deposition process. Multiple blanks are cut from such boules and used to make the various optical members referred to above.
As the energy and pulse rate of lasers increase, the optical members which are used in conjunction with such lasers, are exposed to increased levels of laser radiation. Fused silica members have become widely used as the manufacturing material of choice for optical members in such laser-based optical systems due to their excellent optical properties and resistance to laser induced damage.
Laser technology has advanced into the short wavelength, high energy ultraviolet spectral region, the effect of which is an increase in the frequency (decrease in wavelength) of light produced by lasers. Of particular interest are short wavelength excimer lasers operating in the UV and deep UV (DUV) wavelength ranges, which includes lasers operating at about 248 nm and 193 nm wavelengths, respectively. Excimer laser systems are popular in microlithography applications, and the shortened wavelengths allow for increased line densities in the manufacturing of integrated circuits and microchips, which enables the manufacture of circuits having decreased feature sizes. A direct physical consequence of shorter wavelengths (higher frequencies) is higher photon energies in the beam due to the fact that each individual photon is of higher energy. In such excimer laser systems, fused silica optics are exposed to high energy photon irradiation levels for prolonged periods of time resulting in the degradation of the optical properties of the optical members.
It is known that laser-induced degradation adversely affects the performance of fused silica optical members by decreasing light transmission levels, altering the index of refraction, altering the density, and increasing absorption levels of the glass. Specifically, the two main types of laser-induced degradation that can occur are induced absorption and induced index change as a result of density change.
Given the semiconductor industry reliance on excimer lasers and materials that transmit that energy to make integrated circuit chips and other products and the constant drive towards decreased line width and the necessary wavelength of the incident light and the resultant increase in the laser energy level, it follows that the fused silica material requirements have become much more stringent. The new generation material must be as inert as possible with respect to the incident energy. Many other researchers have tried to make a material that does not interact with the light, however they have been unsuccessful. Accordingly, it would be desirable and is an objective of the present invention to provide fused silica glass articles that exhibited improved resistance to laser induced index/density change.